What constitutes neglect by long time friends?

A friend of mine had a close relationship with several people. These people are also my friends. However, since two years ago, one of the friends have been drawing away from her and paying more attention to the other friends. These friends still have a good relationship with her, but they are not as close as before. This is primarily because they are spending a lot more time with the friend that pulled away from her. He has noticeably found ways to occupy them. Each time she spends with them, he does something to isolate her from being in their company. So basically he has made a circle for himself. I know that people change from time to time. Nevertheless, could this be intrusive, or unintended?